Refining hematite pig iron in a converter

ABSTRACT

A gas consisting mainly of commercial oxygen is blown in by a tuyere or tuyeres in the bottom or side wall of the converter under the surface of the molten metal. At the beginning of the refining operation, a quantity of lime is introduced through the mouth of the converter. The quantity is lower than 12 kg/kg Si in the pig iron. The total surface of the lime is at least 45 m2/t pig iron, and a part of the lime has a specific surface higher than 2.5 m2/kg.

United States Patent Nilles Apr. 30, 1974 REFINING HEMATITE PIG IRON INA2,950,186 I 8/1960 Allard 75/60 CONVERTER 2,855,293 10/1958 Savard 75/601,145,506 7/1915 Pasquier 75/52 Inventor: Paul Emlle Nllles, g,2,918,365 12/1959 Kanamori 75/53 Belgium 2,864,689 12/1958 Perrin 75/59[73] Assign e: C n r De Recherches FOREIGN PATENTS OR APPLICATIONSMetallureiques-Centrum Voor 553 120 4 1956 I 1 75 52 Research In DeMetallurgie ta y Brussels Belglum Primary ExaminerL. DeWayne Rutledge[22] Filed: Feb. 9, 1972 Assistant Examiner-Peter D. Rosenberg [2]] App]No 224 746 Attorney, Agent, or FirmH0lman & Stern [30] ForeignApplication Priority Data [57] ABS CT Feb. 10, 1971 Luxembourg 62570 Agas consisting mainly 0f Commercial Xyge n is Apr 7, 1971 Luxembourg62944 blown in by a tuyere myeres in the bottom or Side wall of theconverter under the surface of the molten 52 us. c1. 75/52, 75/60rnetel- Al the beginning of the refining operation, a 51 Int. Cl. C2lc7/00, 0210 5/34 q y of lime is introduced through the month of 58 Fieldof Search 75/52-60 the eenvener- The quantity is lower than 12 g Si inthe pig iron. The total surface of the lime is at least 5 ReferencesCited 45 m /t pig iron, and a part of the lime has a specific surfacehigher than 2.5 m /kg.

15 Claims, 1 Drawing Figure REFINING HEMATITE PIG IRON IN A CONVERTERThe present invention concerns improvements in methods for refininghematite pig iron in a converter by means of an oxygen-containing gasblown in through the bottom or through the lateral wall of the converterthrough at least one tuyere situated under the surface of the bath ofmetal to be refined.

It is well known that, in refining a hematite pig iron by means of anoxidizing gas, the problem of the correct control of the development ofthe entire'refining operation becomes more difficult to solvesatisfactorily the richer the oxidizing gas is in oxygen. One obviousreason for this is that the local speed of certain reactions and thetemperatures developed at certain places in the bath are in directproportion to this oxygen content.

It is also known that the correct development of a refining operationfor hematite pig iron, particularly in refining by means of commerciallypure oxygen, is bound up with a definitely pre-arranged time developmentof the composition of the slag. Thus, the composition of the slag, forinstance, has a great effect on its participation in the decarburizationreaction and consequently on the development of this reaction.

Moreover, it is known that the spattering of sparks and the overflowingof slag, in themselves quite clearly prejudicial tothe safety of theoperators and to the yield of iron obtained from the refining operation,can be reduced by suitable development of the composition of the slagduring the refining operation; in addition, the viscosity of the slaghas an influence on the tendency of the slag to foam and overflow.Thusit is evidently important to have effective methods for controllingthis development.

When one intends to carry out a refining operation of hematite pig ironby blowing oxygenthrough the bottom or through the lateral wall of theconverter in the conventional manner, one must bear in mind that inpractice there is only one single parameter of the operation that onecan use for control. The reason for this is simple: the oxygen blownunder the surface of the bath of molten metal to be refined penetratesdirectly into the bath before passing in part into the slag. Accordinglythere is no possibility of transferring at will onto the slag a givenproportion of the oxygen blown in, and of acting on its formation. Theonly parameter of operation that one has available is the rate of flowof oxygen blown in, and this can have only a very limited influence onthe distribution of the oxygen between the metal bath and the slag. Thenatural consequence of this state of affairs is that one is forced toaccept the development of the composition of the slag rather than beable to control it, which constitutes a considerable disadvantage.Theabsence of iron in the slag is notably at the source of difficultiesin dissolution of the lime in the slag.

Various attempts have already been made toovercome this disadvantage byattempting to use another variable parameter. It has already beenproposed in particular to put finely divided lime in suspension in therefining gas blown into the bath of metal through the bottom of theconverter. When this was done considerable wear occurred in the tuyeres;furthermore, the transport of the powder and its distribution among thevarious tuyeres gave rise to serious difficulties.

The addition of lime in powder form to the top of a converter duringblowing has always been considered ineffective because the powdered limewas carried away by the converter gases before reaching the bath ofmetal. It was not possible to achieve success without propelling thelime particles at high speed, which is what takes place in top blowingmethods such as the LD-AC process.

It was also proposed to add lime in pieces to the bath of metal throughthe mouth of the converter (the pieces being of sufficient size to reachthe bath without there being any danger of them being carried away bythe converter gases); it was then found that during blowing considerablespatterings of slag and metal occurred, which prejudiced the economy ofthe method and extended the duration of the refining operationunacceptably.

In contrast to what current opinion has considered to be wellestablished, the applicant has found that, in a particularly remarkablemanner and quite unexpectedly, if the refining operation is effectedwith the blowing of commercially pure oxygen through the bottom or thelateral wall of the converter under the surface of the metallic bath,one can rapidly obtain a quite fluid and reactive slag which does notgive rise to any spattering or overflowing, by introducing through themouth of the converter before the start of the refining operation orsoon after the start (preferably before the start)a quantity of lime ofpredetermined granulometry, provided the quantity of steel produced inthe converter is taken into account.

Accordingly, the present invention provides a method of refininghematite pig iron in a converter comprising the refining operation ofblowing in a gas consisting mainly of commercially pure oxygen throughat least one tuyere opening out under the surface of the molten metal,the method including introducing through the mouth of the converter atthe beginning of the refining operation a quantity of lime lower than 12kg per kilogram of silicon initially contained in the pig iron, thegranulometry of the quantity of lime being such that itstotal surface isat least 45 m per metric ton of pig iron, and a part of the quantity oflime being fine lime having a specific surface higher than 2.5m kg.

The best results are obtained when the lime is introduced before thestart of the refining (oxygen blowing) operation, but the results arestill excellent if the addition takes place just after the start of theblowing operation, when the decarburization rate has not yet reached aquarter of its maximum value.

Specific surface means the sum of the external surfaces of all thepieces contained in a kilogram of lime, all the pieces being assumed tobe spherical. In practice, the lime used is assumed to be constituted ofsmall spheres of a diameter equal to the mean granulometry excludingabrasion fines.

The sole FIGURE of the accompanying drawing is a graph of specificsurface against average granulometry.

For limes of known average granulometry the specific surface can bedetermined for instance from the graph shown in the FIGURE, in which yis the specific surface (m /kg) and x is the average granulometry of thelime (mm). The graph plotted is a hyperbola whose equation isapproximately xy 6.

The total surface per metric ton of pig iron is equal to the sum, overall the categories of lime used, of the specific surface multiplied bythe corresponding amount of lime per metric ton of pig iron for eachcategory of lime. For comparison, one can mention that in the LD processit is customary for the lime supplied to be of granulometry 20/40mm,which corresponds to a specific surface of approximately 0.2m /kg; ifone introduces 60 kg of lime per metric ton of pig, iron, the totalsurface is equal to 0.2 X 60 l2m /t pig iron. ln the basic Bessemer orThomas process the customary lime granulometry is /60 which correspondsto a specific surface of 0.l7m /kg.

ln proceeding in accordance with the invention, it is observed, contraryto what has already been taken as fact, that the lime with high specificsurface (more than 2.5m /kg) is not carried away by the converter gases(fumes). On the contrary, under the conditions defined above, one findsthat the assimilation of the very fine lime takes place before the limehas been able to be carried away by the fumes.

In accordance with an advantageous variation of the invention, the limewhich constitutes the fine fraction has an average granulometry notexceeding lmm, the remainder being constituted of pieces of granulometrylarger than that of the lime added at the beginning, and preferably of agranulometry of at least mm. This variation has the advantage of using aconsiderable part of the lime in an economical form.

In accordance with another even more advantageous variation, one usesrich lime, that is lime having a granulometry the major part of whichdoes not exceed 0.02mm. With this variation it is possible to use aminimum quantity of lime with high specific surface.

In the case where particularly fine lime is added before the start ofthe blowing operation it is particularly advantageous to carry out thisaddition by means of lime in bags. 0

Another advantageous variation consists in effecting the deposition ofthe lime in powder form on the pig iron by pneumatic means. Thisvariation is also very suitable for adding, at the beginning of oxygenblowing, lime with a granulometry which is for instance lower than lmm.The powdered lime may be fluidized with air and conducted into the mouthof the converter.

Again, it is advantageous for the quantity of lime of high specificsurface (fine lime) that is lime whose specific surface is greater than2.5m /kg, (which quantity is preferably between 0.5 and 5 kg per kg ofsilicon initially contained in the pig iron) to be linked to thespecific volume, V, of the converter.

By specific volume, V, there should be understood the ratio between theinternal volume of the converter (immediately after construction orre-lining) and the average of steel tapped into the ladle from theconverter per cast (blow).

More specifically, it is recommended that for a given silicon content ofthe pig iron the minimum quantity Q of lime with high specific surfacewill be higher the lower the specific volume is.

This minimum quantity of lime of high specific surface is thusdetermined by an equation of the type: Q =f(i, V), in which i representsthe initial silicon content of the pig iron charged into the furnace.This quantity Q, of fine lime can be greater than Q though under theseconditions the operation will not be so economically bearing in mind thecost of the products.

The ratio P of the quantity Q of lime with high specific surface (finelime) to the total quantity of lime introduced into the furnace isaccordingly a function of the same factors i, V. In general, P 2 V 10 i.

In this equation, P is expressed in percent, V in M /t of steel, and irepresents in weight percent the silicon content of the pig iron.

A particularly interesting example of the method of the invention is onein which: a. refining is effected by means of a gas consisting mainly ofcommercial oxygen, blown into the converter under the surface of thebath (for instance through the walls or the bottom) by means of at leastone tuyere with two coaxial tubes, the internal tube being for theinjection of the refining gas, while the peripheral tube is intended forblowing a gaseous or liquid protective fluid, i.e., a fluid withendothermic dissociation, for instance a hydrocarbon; b. at the verybeginning of the refining operation, and preferably before itscommencement, one introduces through the mouth of the converter aquantity of lime less than 12 kg per kg of silicon initially containedin the pig iron; the granulometry of this lime is such that the totalsurface (assuming the pieces to be spherical) is higher than 45m permetric ton of pig iron, and a portion of this lime (between 0.5 and 5 kgper kg of silicon initially contained in the pig iron) has a specificsurface higher than 2.5m /kg of lime; c. the total quantity of pig iron,scrap, and possibly ores, charged into the converter before thebeginning or during the course of the refining operation is such thatthe specific volume is between 0.40 and 0.90 m metric ton of steelproduced; and d. the ratio P between the quantity of lime with specificsurface higher than 2.5m /kg of lime and the total quantity of limeincreases (for a given silicon content of the pig iron) as the specificvolume of the ladle decreases.

It has for instance been found that condition ((1) can be expressed bythe relationship where V is expressed in m /t of steel obtained in irepresents the silicon content of the pig iron in weight percent.

It should be pointed out here that the method does not involve theinjection of lime directly into or onto the bath of molten metal.

As already mentioned above, it has surprisingly and unpredictably beenfound that a hematite pig iron refining operation carried out as aboveenables a fluid and reactive slag to be obtained rapidly withoutspattering or overflowing, and consequently a quality steel underadvantageous conditions of time and economy.

The addition of lime to the converter in the form of pieces can beeffected in the conventional manner, for instance by means of a hopper,the addition of lime of granulometry below 2 mm preferably taking placeeither in sacks or by pneumatic means, excluding any injection of limeonto or into the metal.

The lime introduced before the blowing operation or at the beginning ofthe blowing operation, can be mixed with constituents such as fluidizers(spath fluor, alumina), cooling agents (ores, soda, scale), and so on,these constituents preferably having a granulometry similar to that ofthe lime.

One can also add compounds which melt more easily than the lime, inparticular lime ferrites, to take the place of all or a part of thelime.

The granulometry of the lime ferrites will correspond to the indicationsgiven in.the above description. Example A.

A converter of 11.3 in useful volume was charged with 14.6 t (metricton) of pig iron containing 0.55% Si, 3.2 t scrap, and 800 kg lime madeup as follows:

63 percent (i.e., 504 kg lime) in powder form of average granulometry 1mm (specific surface: 6m /kg), and

37 percent (i.e., 296 kg lime) in pieces of granulometry 20/40 mm(specific surface: approximately 0.2 m /kg).

The yield was 16.4 t good quality steel, with no losses or spattering.

Thus the quantity of lime introduced before the start or at the start ofthe blowing operation represents 504 kg 296 kg/l4.6 X 5.5 kg lime per kgsilicon initially contained in the pig iron; the first condition issatisfied 12 kg).

The total surface of the lime is 0.2 m X 296 6 m X 504/ 14.6 or 211 m /tpig iron, satisfying the second condition 45 m P 130 150 V 10 i is alsosatisfied:

63 2 130 150 X l1.3/16.4 l 10 X 055 as well as the relation 0.40 V' 0.90m lt steel produced since V 0.69.

The minimum proportion of fine lime to be added to obtain oxygenblowingwithout spattering would have been 32 percent, i. e 800 X 32/100= 256 kg or 3.2 kg per kg silicon initially contained in the pigiron; this minimum quantity is within the preferable range of 0.5 to 5kg per kg silicon mentioned above.

Example B The same converter was charged with 18.5 t hematite pig ironwith 0.80 percent silicon, 4.9 t scrap, and 1,400 kg lime: 700 kg inpieces (20/40 mm) and 700 kg in powder of average granulometry 2 mm(specific surface: 3 m /kg). The yield was 21.5 metric tons of steel butthere 'was some spattering.

By calculating the various conditions one obtains the following results:

a. 700 700/18.5 X 8 9.5 kg of lime per kg silicon initially contained inthe pig-iron (i.e. 12); ..2 m X 111 8 3 m2 X 700 e121 mute).

c. the specific surface of the fine lime is 3 m lkg (i.e., 2.5), thequantity of lime of this category being 4.7 kg/kg Si (i.e., within thepreferred range);

d. the relation P( =50) 130 150 V+ 10 i is not at sfied hecayse; 1.9-..150 ...1.1 1. /2.1; $128918? ing in a gas consisting essentially ofcommercially pure oxygen through at least one tuyere opening out underthe surface of the molten metal, the method including depositing on thesurface of the molten metal through the mouth of the converter aquantity of lime which is between 0.5 and 12 kg per kilogram of siliconinitially contained in the charge of pig iron, the granulometry of thequantity of lime being such that its total surface is at least 45m permetric ton of pig iron, and a part of the quantity of lime in the amountof 0.5 to 5 kg per kilogram of silicon initially contained in the pigiron being fine lime having a specific surface greater than 2.5m /kg oflime, the quantity of lime being introduced after the molten metal hasbeen introduced and before the decarburization rate has reachedone-quarter of its maximum value.

2. A method as claimed in claim 1, in which the quantity of lime isintroduced before the start of blowing.

3. A method as claimed in claim 1, in which the fine lime has an averagegranulometry of at most 1 mm, the remainder of the quantity of limeconsisting of pieces having a larger granulometry than that of the finelime.

4. A method as claimed in claim 3, in which the granulometry of theremainder is at least 15 mm.

5. A method as claimed in claim 1, in which the quantity of limecomprises rich lime having a granulometry the major part of which doesnot exceed 0.02 mm.

6. A method as claimed in claim 1, including introducing the quantity oflime in bags.

7. A method as claimed in claim 1, including introducing the quantity oflime pneumatically.

8. A method as claimed in claim 1, in which the amount of fine lime is afunction of the specific volume of the converter.

9. A method as claimed in claim 8, including increasing the amount offine lime when the specific volume is reduced, and decreasing the amountwhen the specific volume is increased.

10. A method as claimed in claim 1, in which the proportion of fine limein the said quantity of lime obeys the following relationship:

where P is the proportion of fine lime in the said quantity of lime, inpercent by weight, V is the specific volume of the converter in mlmetric ton of steel produced, and i is the silicon content of the pigiron charged into the converter, in percent by weight.

11. A method as claimed in claim '1, in which at least part of the limeis introduced in the form of a lime compound which melts more easilythan uncombined lime.

12. A method as claimed in claim 11, in which the compound is a limeferrite.

13. A method as claimed in claim 1, in which the tuyere comprises aninner tube and an outer tube encompassing the inner tube, the methodincluding injecting the oxygen-containing gas through the inner tube andinjecting a fluid which dissociates endothermically, through the outertube.

14. A method as claimed in claim 1, including introducing into theconverter at least one fluidizer selected from the group consisting ofspath fluor and alumina.

15. A method as claimed in claim 1, including introducing into theconverter at least one cooling agent selected from the group consistingof iron ore, soda and scale.

2. A method as claimed in claim 1, in which the quantity of lime is introduced before the start of blowing.
 3. A method as claimed in claim 1, in which the fine lime has an average granulometry of at most 1 mm, the remainder of the quantity of lime consisting of pieces having a larger granulometry than that of the fine lime.
 4. A method as claimed in claim 3, in which the granulometry of the remainder is at least 15 mm.
 5. A method as claimed in claim 1, in which the quantity of lime comprises rich lime having a granulometry the major part of which does not exceed 0.02 mm.
 6. A method as claimed in claim 1, including introducing the quantity of lime in bags.
 7. A method as claimed in claim 1, including introducing the quantity of lime pneumatically.
 8. A method as claimed in claim 1, in which the amount of fine lime is a function of the specific volume of the converter.
 9. A method as claimed in claim 8, including increasing the amount of fine lime when the specific volume is reduced, and decreasing the amount when the specific volume is increased.
 10. A method as claimed in claim 1, in which the proportion of fine lime in the said quantity of lime obeys the following relationship: P > or = 130 - 150 V + 10i, where P is the proportion of fine lime in the said quantity of lime, in percent by weight, V is the specific volume of the converter in m3/metric ton of steel produced, and i is the silicon content of the pig iron charged into the converter, in percent by weight.
 11. A method as claimed in claim 1, in which at least part of the lime is introduced in the form of a lime compound which melts more easily than uncombined lime.
 12. A method as claimed in claim 11, in which the compound is a lime ferrite.
 13. A method as claimed in claim 1, in which the tuyere comprises an inner tube and an outer tube encompassing the inner tube, the method including injecting the oxygen-containing gas through the inner tube and injecting a fluid which dissociates endothermically, through the outer tube.
 14. A method as claimed in claim 1, including introducing into the converter at least one fluidizer selected from the group consisting of spath fluor and alumina.
 15. A method as claimed in claim 1, including introducing into the converter at least one cooling agent selected from the group consisting of iron ore, soda and scale. 